Circuit for Interconnected Direct Current Power Sources

ABSTRACT

Controlling a power converter circuit for a direct current (DC) power source is disclosed. The power converter may be operative to convert input power received from the DC power source to an output power and to perform maximum power point tracking of the power source. The power converter is adapted to provide the output power to a load that also performs maximum power point tracking.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/722,479, filed on Apr. 18, 2022, which is a continuation of U.S.application Ser. No. 16/899,107 (now U.S. Pat. No. 11,334,104), filed onJun. 11, 2020, which is a continuation of U.S. application Ser. No.15/831,850 (now U.S. Pat. No. 10,705,551), filed on Dec. 5, 2017, whichis a continuation of U.S. application Ser. No. 14/401,049 (now U.S. Pat.No. 9,870,016), filed on Nov. 13, 2014, which was a U.S. National StageApplication of PCT Application No. PCT/US13/42354, filed on May 23,2013, which claims priority to U.S. Provisional Application No.61/651,834, filed on May 25, 2012. These disclosures are incorporatedherein by reference in their entireties.

BACKGROUND

Embodiments described in this application relate generally to control ofpower production from distributed current sources such as direct current(DC) power sources.

Recent interest in renewable energy has led to increased research insystems for distributed generation of energy, such as photovoltaic cells(PV), fuel cells and batteries. Various inconsistencies in manufacturingmay cause two otherwise identical sources to provide different outputcharacteristics. Similarly, two such sources may react differently tooperating conditions, e.g. load and/or environmental conditions, e.g.temperature. In installations, different sources may also experiencedifferent environmental conditions, e.g., in solar power installationssome panels may be exposed to full sun, while others may be shaded,thereby delivering different power output. In a multiple batteryinstallation, some of the batteries may age differently, therebydelivering different power output.

BRIEF SUMMARY

Various embodiments relate to power conversion in a distributed energysystem that may have some of characteristics described above. While thevarious embodiments may be applicable to any distributed power system,the following discussion turns to solar energy so as to provide a betterunderstanding by way of example without limitation to otherapplications.

Distributed power systems are described, including a power convertercircuit for a direct current (DC) power source such as one or morephotovoltaic panels, photovoltaic substrings or photovoltaic cells. Aload, e.g. grid-tied inverter, may be connected by DC power lines toreceive the harvested power from one or more of the power convertercircuits. According to an aspect, the power converter circuit mayinclude a direct current to direct current (DC/DC) power converterconfigured to convert DC power received on a DC/DC power converter inputfrom the photovoltaic panel(s) to a DC/DC power converter output. Thecircuit may include a control circuit, which is configured to senseinput voltage and/or input current and to determine input power receivedon the DC/DC power converter input (output power from the photovoltaicpanel). The control circuit may be configured to maximize the inputpower by operating the power source (e.g., photovoltaic panel) at acurrent and voltage that is tracked to maximize the power yield of thepower source, or its maximum power point. Since the maximum power pointtracking is performed at the input of the power converter, the outputvoltage or current of the power converter is not fully constrained.While the power output from the DC/DC converter is about equal to theinput power from the photovoltaic power times the efficiency of theconversion, the voltage and current at the output of the DC/DC powerconverter may be set, determined and/or controlled by the load or by acontrol circuit at the input of the load. The load may be an inverteradapted to convert the DC power to alternating current (AC) at thefrequency of the grid. According to an aspect, the inverter does notutilize a maximum power point tracking (MPPT) module since the maximumpower from each DC source is already tracked individually for each panelby the control circuits. The inverter may have a control block at itsinput which sets the input voltage at a convenient value, optionally apredetermined value, and/or optionally a constant value, e.g. 400 Volts,for instance to maximize the efficiency of the load, e.g. inverter, orto minimize power loss in the DC lines.

However, many commercially available inverter modules already includeintegrated MPPT tracking circuits designed for use with conventionalphotovoltaic distributed power systems that do not include individualMPPT tracking for each power source as described above. It would bedesirable that standard commercially available inverters with integratedMPPT modules be compatible with the DC/DC power converter circuits withthe control circuits, which individually maximize power from the DCpower sources, e.g. photovoltaic panels. However, since the controlcircuit maintains the photovoltaic panel at its maximum power point, thepower output of the DC/DC converters may not present to the input of theinverter a power peak that can be tracked by the inverter's integratedMPPT as current or voltage at the output of the DC/DC converter varies.As a result, an MPPT module, if present at the inverter input may not beable to stabilize and lock onto any particular voltage that maximizespower at the input to the inverter. As a result, the MPPT module of theinverter is used in a system according to aspects may force the input tothe inverter to an extreme voltage (or current), and/or become unstableand considerable power may be lost.

Thus, there is a need for and it would be advantageous to have powerconverter circuits which operate universally with all or most types ofinverters whether equipped with an MPPT module or not and for a loadequipped with a control block which sets input voltage to the load to aconvenient optionally constant value as described above. Variousmethods, systems and/or devices are disclosed herein, which provide apower converter circuit including a power converter connectible to adirect current (DC) power source such as a photovoltaic panel. Thedirect current (DC) power source may include one or more photovoltaicsolar cells or solar panels interconnected in series and/or in parallel.The power converter includes input terminals adapted for connecting tothe direct current (DC) power source and output terminals. The powerconverter may be operative to convert input power received from the DCpower source at the power converter input terminals to an output powerat the power converter output terminals. The power converter may have acontrol circuit connected at the power converter input terminals so thatduring operation of the power converter, the control circuit sets theinput voltage or the input current at the power converter inputterminals to maximize the input power, e.g., to perform maximum powerpoint tracking (MPPT). A maximum power point tracking circuit may alsobe connected to the power converter output terminals. The powerconverter may include multiple like power converter circuits seriesconnected at their output terminals into serial strings. The serialstrings may be parallel connected and input to the load via the maximumpower point tracking circuit. The having load input terminals and loadoutput terminals may be configured to receive power from the powerconverter, e.g., via the maximum power point tracking circuit connectedto the power converter output terminals. The load may be an inverter ora DC/DC power converter.

According to different features:

-   -   A. The output voltage of the power converter may be sensed. The        control circuit may be configured to set the input power        received at the input terminals of the power converter to a        maximum power only at a predetermined output voltage point or        output voltage range or at a predetermined output current point        or output current range. Away from the predetermined output        voltage or predetermined output current, the control circuit may        be configured to set the input power received at the input        terminals to less than the maximum available power.    -   a. In this way, the maximum power point tracking circuit        operatively connected to the output terminals of the power        converter may stably track the predetermined voltage and/or        current point or range.    -   B. The control circuit may be configured to set the input power        received at the input terminals to the power converter to a        maximum power. A power attenuator may be connected to the output        terminals of the power converter. The power attenuator may be        configured to attenuate power output at output voltages other        than at a predetermined output voltage range (or a predetermined        output current range) and not to attenuate output power at the        predetermined output voltage or current point or range. The        maximum power point tracking circuit may be connected to the        attenuated power output. The maximum power point tracking        circuit may be configured to lock onto the maximum power point        at the predetermined output voltage range or at the        predetermined output current range. The load may be typically        configured for receiving power from the power converter via the        power attenuator and via the maximum power point tracking        circuit connected to the attenuated power output.    -   C. The control circuit may be configured to set the input power        received at the input terminals of the power converter to the        maximum power point of the power source. A control circuit        connected to the input terminals is configured to vary the        voltage conversion ratio defined as the ratio of input voltage        to output voltage of the power converter. The voltage conversion        ratio may be varied or perturbed to slowly approach maximum        power on the output terminals. The term “slowly” as used herein        is relative to the response time of the MPPT circuit associated        with the load (e.g., at the output of the power converter). The        conversion ratio may be selected to achieve maximum power. Since        the output power from the power converter approaches slowly        maximum power, the MPPT circuit associated with the load        responds accordingly and locks onto the predetermined output        voltage at maximum output power.    -   D. The maximum power point tracking circuit associated with the        load during the course of its operation may perturb its voltage        or current input (output to the power converter). The power        converter may include a control circuit to set the input power        received at the input terminals of the power converter to the        maximum power point and a control circuit configured to sense        output voltage. The conversion ratio of the power conversion is        slowly varied by the control circuit to slowly approach the        selected conversion ratio and the predetermined output voltage        at the maximum power point.    -   E. The features of paragraphs C and D are not exclusive and may        be used in combination. If a change in output voltage at the        output of the power converter is sensed then the conversion        ratio of the power conversion is slowly varied by the control        circuit to slowly approach the selected conversion ratio and the        predetermined output voltage. Otherwise if a substantial change        in output voltage is not sensed, the control circuit is        configured to vary the output voltage to slowly approach the        desired conversion ratio while the MPPT circuit approaches the        maximum power point

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example and not limitedin the accompanying figures in which like reference numerals indicatesimilar elements and in which:

FIG. 1 illustrates a conventional centralized power harvesting systemusing DC power sources;

FIG. 2 illustrates current versus voltage characteristic curves for oneserial string of DC sources;

FIG. 3 illustrates a distributed power harvesting system, according toembodiments, using

a. DC power sources;

FIGS. 4A and 4B illustrate the operation of the system of FIG. 3 underdifferent conditions, according to embodiments;

FIG. 4C illustrates a distributed power harvesting system, according toembodiments, wherein the inverter controls the input current;

FIG. 5 illustrates a distributed power harvesting system, according toother embodiments, wherein the voltage at the input of the inverter iscontrolled;

FIG. 6 illustrates an exemplary DC-to-DC converter according toembodiments;

FIG. 7 illustrates a power converter including control featuresaccording to various embodiments;

FIG. 8A illustrates graphically behavior of power output from solarpanels as a function of output current in a conventional system;

FIG. 8B illustrates graphically power input or output versus outputcurrent from one photovoltaic module or a system of series/parallelconnected photovoltaic modules and/or strings;

FIG. 8C illustrates in a block diagram of a distributed power harvestingsystem according to various embodiments;

FIG. 8D illustrates graphically power output as a function of currentmodified according to various embodiments;

FIG. 8E illustrates a circuit for modifying output power according tovarious embodiments;

FIG. 8F illustrates a process of power conversion and tracking maximumpower, according to various embodiments;

FIG. 8G which illustrates a process for operating an inverter equippedwith an MPPT module according to various embodiments;

FIG. 9 illustrates in a simplified block diagram of a distributed powerharvesting system according to various embodiments;

FIG. 9A and FIG. 9B illustrate processes performed in parallel at thepower source and at the maximum power point tracking circuit,respectively, according to various embodiments;

FIG. 9C illustrates graphically variation of power output from one ormore photovoltaic modules as a function to time, according to variousembodiments;

FIG. 10A and FIG. 10B illustrate processes performed in parallel at thephotovoltaic module and maximum power point tracking circuit,respectively, according to various embodiments.

The foregoing and/or other aspects will become apparent from thefollowing detailed description when considered in conjunction with theaccompanying drawing figures.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to the like elements throughout. The embodiments aredescribed below to explain examples by referring to the figures.

A conventional installation of solar power system 10 is illustrated inFIG. 1 . Since the voltage provided by each individual solar panel 101may be low, several panels may be connected in series to form a stringof panels 103. For a large installation, when higher current may beutilized, several strings 103 may be connected in parallel to form theoverall system 10. Solar panels 101 may be mounted outdoors, and theirleads may be connected to a maximum power point tracking (MPPT) module107 and then to an inverter 104. The MPPT 107 may be implemented as partof the inverter 104.

The harvested power from the DC sources may be delivered to the inverter104, which converts the fluctuating direct-current (DC) intoalternating-current (AC) having a desired voltage and frequency at theinverter output, which may be, e.g., I IOV or 220V at 60 Hz, or 220V at50 Hz. In some examples, inverters that produce 220V may be then splitinto two I IOV feeds in an electric box. The AC current from theinverter 104 may then be used for operating electric appliances or fedto the power grid. Alternatively, if the installation is not tied to thegrid, the power extracted from inverter 104 may be directed to aconversion and charge/discharge circuit to store the excess powercreated as charge in batteries. In case of a battery-tied application,the inversion stage might be skipped altogether, and the DC output ofthe MPPT stage 107 may be fed into the charge/discharge circuit.

As noted above, each solar panel 101 supplies relatively very lowvoltage and current. A challenge facing the solar array designer may beto produce a standard AC current at 120V or 220V root-mean-square (RMS)from a combination of the low voltages of the solar panels. The deliveryof high power from a low voltage may utilize very high currents, whichmay cause large conduction losses on the order of the second power ofthe current (IQ). Furthermore, a power inverter, such as the inverter104, which may be used to convert DC current to AC current, may be mostefficient when its input voltage may be slightly higher than its outputRMS voltage multiplied by the square root of 2. Hence, in manyapplications, the power sources, such as the solar panels 101, may becombined in order to reach the correct voltage or current. A commonmethod may be to connect the power sources in series in order to reachthe desirable voltage and in parallel in order to reach the desirablecurrent, as shown in FIG. 1 . A large number of the panels 101 may beconnected into a string 103 and the strings 103 may be connected inparallel to the power inverter 104. The panels 101 may be connected inseries in order to reach the minimal voltage for the inverter. Multiplestrings 103 may be connected in parallel into an array to supply highercurrent, so as to enable higher power output.

While this configuration may be advantageous in terms of cost andarchitecture simplicity, several drawbacks have been identified for sucharchitecture. One drawback may be inefficiencies caused by non-optimalpower draw from each individual panel, as explained below. The output ofthe DC power sources may be influenced by many conditions. Therefore, tomaximize the power draw from each source, one may need to draw thecombination of voltage and current that provides the peak power for thecurrently prevailing conditions of the power source. As conditionschange, the combination of voltage and current draw may need to bechanged as well.

FIG. 2 illustrates an example of one serial string of DC sources, e.g.,solar panels 101 a 101 d, and MPPT circuit 107 integrated with inverter104. The current versus voltage (IV) characteristics are plotted (210a-210 d) to the left of each DC source 101. For each DC source 101, thecurrent decreases as the output voltage increases. At some voltagevalue, the current goes to zero, and in some applications may assume anegative value, meaning that the source becomes a sink. Bypass diodesmay be used to prevent the source from becoming a sink. The power outputof each source 101, which may be equal to the product of current andvoltage (P=I*V), varies depending on the voltage across the source. At acertain current and voltage, close to the falling off point of thecurrent, the power reaches its maximum. It may be desirable to operate apower generating power source (e.g., photovoltaic panel, cell, etc.) atthis maximum power point. The purpose of the MPPT may be to find thispoint and operate the system at this point to draw the maximum powerfrom the sources.

In a typical, conventional solar panel array, different algorithms andtechniques may be used to optimize the integrated power output of thesystem 10 using the MPPT module 107. The MPPT module 107 may receive thecurrent extracted from all of the solar panels together and may trackthe maximum power point for this current to provide the maximum averagepower such that if more current is extracted, the average voltage fromthe panels starts to drop, thus lowering the harvested power. MPPTmodule 107 maintains a current that yields the maximum average powerfrom the overall system 10. However, since sources 101 a-101 d may beconnected in series to a single MPPT 107, the MPPT may select a singlepower point, which would be somewhat of an average of the maximum powerpoints (MPP) of each of the serially connected sources. In practice, itmay be very likely that the MPPT would operate at an I-V point that maybe optimum to only a few or none of the sources. In the example of FIG.2 , each of the sources operate at the same current since the sourcesare connected in series, but the maximum power point for each source(indicated by a dot on curves 210 a-210 d) may be at different currents.Thus, the selected current operating point by MPPT 107 may be themaximum power point for source 101 b, but may be off the maximum powerpoint for sources 101 a, 101 c and 101 d.

-   -   a. Consequently, the arrangement may be not operated at best        achievable efficiency.

Turning back to the example of system 10 of FIG. 1 , fixing apredetermined constant output voltage from the strings 103 may causesolar panels 101 to supply lower output power than otherwise possible.Further, each string 103 carries a single current that is passed throughall of solar panels 101 along string 103. If solar panels 101 aremismatched due to manufacturing differences, aging or if theymalfunction or placed under different shading conditions, the current,voltage and power output of each panel may be different. Forcing asingle current through all of panels 101 of string 103 may causeindividual panels 101 to work at a non-optimal power point and can alsocause panels 101, which may be highly mismatched to generate “hot spots”due to the high current flowing through them. Due to these and otherdrawbacks of conventional centralized methods of MPPT, panels 101 may bematched improperly. In some cases, external diodes may be used to bypasspanels 101 that are highly mismatched. In conventional multiple stringconfigurations all strings 103 may be composed of exactly the samenumber of solar panels and panels 101 may be selected of the same modeland may be installed at exactly the same spatial orientation, beingexposed to the same sunlight conditions at all times. Installationaccording to these constraints may be very costly. During installationof a solar array according to the conventional configurations 10, theinstaller can verify the correctness of the installation and performanceof the solar array by using test equipment to check the current-voltagecharacteristics of each panel, each string and the entire array. Inpractice, however, individual panels and strings may be either nottested at all or tested only prior to connection. Current measurementmay be performed by a series connection to the solar array such as witha series resistor in the array, which is typically not convenient.Instead, typically only high-level pass/fail testing of the overallinstallation is performed.

After the initial testing of the installation, the solar array may beconnected to inverter 104, which may include a monitoring module, whichmonitors performance of the entire array. The performance informationgathered from monitoring within inverter 104 may include integratedpower output of the array and the power production rate, but theinformation lacks any fine details about the functioning of individualsolar panels 101. Therefore, the performance information provided bymonitoring at the inverter 104 may be insufficient to understand ifpower loss may be due to environmental conditions, from malfunctions orfrom poor installation or maintenance of the solar array. Furthermore,integrated information may not pinpoint which of solar panels 101 areresponsible for a detected power loss.

FIG. 3 illustrates a distributed power harvesting configuration 30,according to an embodiment. Configuration 30 enables connection ofmultiple power sources, for example, solar panels 101 a-101 d, into asingle power supply. In one aspect, the series string of all of thesolar panels may be coupled to an inverter 304. In another aspect,several serially connected strings of solar panels may be connected to asingle inverter 304. The inverter 304 may be replaced by other elements,such as, e.g., a charging regulator for charging a battery bank.

In configuration 30, each solar panel 101 a-101 d may be connected to aseparate power converter circuit 305 a-305 d. One solar panel 101together with its connected power converter circuit forms a module,e.g., photovoltaic module 302 (only one of which is labeled). Eachconverter 305 a-305 d adapts optimally to the power characteristics ofthe connected solar panel 101 a-101 d and transfers the powerefficiently from converter input to converter output. The converters 305a-305 d may be buck converters, boost converters, buck/boost converters,flyback or forward converters, etc. The converters 305 a-305 d may alsocontain a number of component converters, for example a serialconnection of a buck and a boost converter.

Each converter 305 a-305 d may include a control circuit 311 thatreceives a feedback signal, not from the converter's output current orvoltage, but rather from the converter's input coming from the solarpanel 101. An input sensor measures an input parameter, input power,input current and/or input voltage and sets the input power. An exampleof such a control circuit may be a maximum power point tracking (MPPT)circuit. The MPPT circuit of the converter locks the input voltage andcurrent from each solar panel I Ola-I Old to its optimal power point. Inthe converters 305 a-305 d, according to aspects, a controller withinconverter 305 monitors the voltage and current at the converter inputterminals and determines the pulse width modulation (PWM) of theconverter in such a way that maximum power may be extracted from theattached panel 101 a-101 d. The controller of the converter 305dynamically tracks the maximum power point at the converter input. Invarious aspects, the feedback loop of control circuit 311 may be closedon the input power in order to track maximum input power rather thanclosing the feedback loop on the output voltage as performed byconventional DC-to-DC voltage converters (e.g., MPPT 107). As a resultof having a separate control circuit 311 in each converter 305 a-305 d,and consequently for each solar panel 101 a-101 d, each string 303 insystem 30 may have a different number or different brand of panels 101a-101 d connected in series. Control circuit 311 of FIG. 3 continuouslymaximizes power on the input of each solar panel 101 a-101 d to react tochanges in temperature, solar radiance, shading or other performancefactors that impact that particular solar panel 101 a-I Old. As aresult, control circuit 311 within the converters 305 a-305 d harveststhe maximum possible power from each panel 101 a-101 d and transfersthis power as output power regardless of the parameters impacting theother solar panels.

As such, the embodiments shown in FIG. 3 continuously track and maintainthe input current and the input voltage to each converter 305 at themaximum power point of the connected DC power source. The maximum powerof the DC power source that may be input to converter 305 may be alsooutput from converter 305. The converter output power may be at acurrent and voltage different from the converter input current andvoltage. While maintaining the total power given the minor power lossdue to inefficiency of the power conversion, the output current andoutput voltage from converter 305 may be responsive to requirements ofthe series connected portion of the circuit.

In one embodiment, the outputs of converters 305 a-305 d may be seriesconnected into a single DC output that forms the input to the load, inthis example, inverter 304. The inverter 304 converts the seriesconnected DC output of the converters into an AC power supply. The load,in this case inverter 304, may regulate the voltage at the load's inputusing control circuit 320. That may be, in this example, an independentcontrol loop 320 which may hold the input voltage at a predetermined setvalue, e.g. 400 volts. Consequently, input current of inverter 304 maybe dictated by the available power, and this may be the current thatflows through all serially connected DC sources. While the output of theDC-DC converters 305 are constrained by current and or voltageregulation at the input of inverter 304, the current and voltage inputto power converter circuit 305 may be independently controlled usingcontrol circuit 311. Aspects provide a system and method for combiningpower from multiple DC power sources 101 into a distributed powersupply. According to these aspects, each DC power source 101, e.g.photovoltaic panel 101 may be associated with a DC-DC power converter305. Modules formed by coupling the DC power sources 101 to theirassociated converters 305 may be coupled in series to provide a stringof modules. The string of modules may be then coupled to inverter 304having its input voltage fixed. A maximum power point control circuitcontrol circuit 311 in each converter 305 harvests the maximum powerfrom each DC power source 101 and transfers this power as output frompower converter 305. For each converter 305, the input power may beconverted to the output power, such that the conversion efficiency maybe

-   -   a. 95⁰0 or higher in some situations.

Further, the controlling may be performed by fixing the input current orinput voltage of the converter to the maximum power point and allowingoutput voltage of the converter to vary. For each power source 101, oneor more sensors may monitor the input power level to the associatedconverter 305. In some embodiments, a microcontroller may perform themaximum power point tracking and control in each converter 305 by usingpulse width modulation to adjust the duty cycle used for transferringpower from the input to the output. An aspect may provide a greaterdegree of fault tolerance, maintenance and serviceability by monitoring,logging and/or communicating the performance of each solar panel. Invarious embodiments, the microcontroller that may be used for maximumpower point tracking may also be used to perform the monitoring, loggingand communication functions. These functions allow for quick and easytroubleshooting during installation, thereby significantly reducinginstallation time. These functions may be also beneficial for quickdetection of problems during maintenance work. Aspects allow easylocation, repair, or replacement of failed solar panels. When repair orreplacement may be not feasible, bypass features provide increasedreliability. In an aspect, arrays of solar cells are provided where thepower from the cells may be combined. Each converter 305 may be attachedto a single solar cell, or a plurality of cells connected in series, inparallel, or both, e.g., parallel connection of strings of seriallyconnected cells.

In an embodiment, each converter 305 may be attached to one or morepanels of a photovoltaic string. However, while applicable in thecontext of solar power technology, the aspects may be used in anydistributed power network using DC power sources. For example, they maybe used in batteries with numerous cells or hybrid vehicles withmultiple fuel cells on board. The DC power sources may be solar cells,solar panels, electrical fuel cells, electrical batteries, and the like.Further, although the discussion below relates to combining power froman array of DC power sources into a source of AC voltage, the aspectsmay also apply to combining power from DC sources into another DCvoltage.

In these DC-to-DC voltage converters, a controller within the convertermay monitor the current or voltage at the input, and the voltage at theoutput. The controller may also determine the appropriate pulse widthmodulation (PWM) duty cycle to fix the output voltage to thepredetermined value by increasing the duty cycle if the output voltagedrops. Accordingly, the conventional converter may include a feedbackloop that closes on the output voltage and uses the output voltage tofurther adjust and fine-tune the output voltage from the converter. As aresult of changing the output voltage, the current extracted from theinput may be also varied.

FIGS. 4A and 4B illustrate an operation of the system of FIG. 3 underdifferent conditions, according to embodiments. An exemplaryconfiguration 40 may be similar to configuration 30 of FIG. 3 . In theexample shown, ten DC power sources 101/I through 101/10 may beconnected to ten power converters 305/I through 305/10, respectively.The modules formed by the DC power sources 101 and their connectedconverters 305 may be coupled together in series to form a string 303.In one embodiment, the series-connected converters 305 may be coupled toa DC-to-AC inverter 404.

DC power sources may be solar panels 101 and the example may bediscussed with respect to solar panels as one illustrative case. Eachsolar panel 101 may have a different power output due to manufacturingtolerances, shading, or other factors. For the purpose of the presentexample, an ideal case may be illustrated in FIG. 4A, where efficiencyof the DC-to-DC conversion may be assumed to be 100% and the panels 101may be assumed to be identical. In some aspects, efficiencies of theconverters may be quite high and range at about 95%-99%. So, theassumption of 100% efficiency may not be unreasonable for illustrationpurposes. Moreover, according to embodiments, each of the DC-DCconverters 305 may be constructed as a power converter, i.e., ittransfers to its output the entire power it receives in its input withvery low losses. Power output of each solar panel 101 may be maintainedat the maximum power point for the panel by a control loop 311 withinthe corresponding power converter 305. In the example shown in FIG. 4A,all of panels 101 may be exposed to full sun illumination and each solarpanel 101 provides 200 W of power. Consequently, the MPPT loop may drawcurrent and voltage level that will transfer the entire 200 W from thepanel to its associated converter 305. That is, the current and voltagedictated by the MPPT form the input current I in and input voltage V into the converter. The output voltage may be dictated by the constantvoltage set at the inverter 404, as will be explained below. The outputcurrent Tout would then be the total power, i.e., 200 W, divided by theoutput voltage Vout.

Referring back to conventional system 10, Figures I and 2, the inputvoltage to load 104 varies according to the available power. Forexample, when a lot of sunshine may be available in a solarinstallation, the voltage input to inverter 104 can vary even up to 1000volts. Consequently, as sunshine illumination varies, the voltage varieswith it, and the electrical components in inverter 104 (or other powersupplier or load) may be exposed to varying voltage. This tends todegrade the performance of the components and may ultimately cause themto fail. On the other hand, by fixing or limiting the voltage or currentto the input of the load or power supplier, e.g., inverter 304, theelectrical components may always be exposed to the same voltage orcurrent and possibly have extended service life. For example, thecomponents of the load (e.g., capacitors, switches and coil of theinverter) may be selected so that at the fixed input voltage or currentthey operate at, say, 60% of their rating. This may improve thereliability and prolong the service life of the component, which may becritical for avoiding loss of service in applications such as solarpower systems.

As noted above, according to an embodiment, the input voltage toinverter 404 may be controlled by inverter 404 (in this example, keptconstant), by way of control loop 420 (similar to control loop 320 ofinverter 304 above). For the purpose of this example, assume the inputvoltage may be kept as 400V (ideal value for inverting to 220 VAC).Since it is assumed that there may be ten serially connected powerconverters, each providing 200 W, the input current to the inverter 404is 2000 W/400V=5 A. Thus, the current flowing through each of theconverters 101/I-101/10 may be 5 A. This means that in this idealizedexample each of converters 101 provides an output voltage of 200W/5A=40V. Now, assume that the MPPT for each panel 101 (assuming perfectmatching panels) dictates that the maximum power point voltage for eachpanel is Vmpp 32V. This means that the input voltage to inverter 404would be 32V, and the input current would be 200 W/32V=6.25 A.

We now turn to another example, where system 40 may be still maintainedat an ideal mode (i.e., perfectly matching DC sources and entire powermay be transferred to inverter 404), but the environmental conditionsmay different for different panels. For example, one DC source may beoverheating, may be malfunctioning, or, as in the example of FIG. 4B,the ninth solar panel 101/9 may be shaded and consequently produces only40 W of power. Since all other conditions as in the example of FIG. 4Aare kept, the other nine solar panels 101 may be unshaded and stillproduce 200 W of power. The power converter 305/9 includes MPPT tomaintain the solar panel 101/9 operating at the maximum power point,which may be now lowered due to the shading.

The total power available from the string may be now 9×200 W+40 W=1840W. Since the input to inverter 404 may be still maintained at 400V, theinput current to inverter 404 will now be 1840 W/40V=4.6 A. This meansthat the output of all of the power converters 305/1-305/10 in thestring may be at 4.6 A. Therefore, for the nine unshaded panels, theconverters will output 200 W/4.6 A=43.5V. On the other hand, theconverter 305/9 attached to the shaded panel 101/9 will output 40 W/4.6A=8.7V. Checking the math, the input to inverter 404 can be obtained byadding nine converters providing 43.5 V and one converter providing8.7V, i.e., (9×43.5V)+8.7V=400V.

The output of the nine non-shaded panels would still be controlled bythe MPPT as in FIG. 4A, thereby standing at 32V and 6.25 A. On the otherhand, since the ninth panel 101/9 is shaded, assume its MPP voltagedropped to 28V. Consequently, the output current of the ninth panel is40 W/28V=1.43 A. As can be seen by this example, all of the panels maybe operated at their maximum power point, regardless of operatingconditions. As shown by the example of FIG. 4B, even if the output ofone DC source drops dramatically, system 40 still maintains relativelyhigh power output by fixing the voltage input to the inverter, andcontrolling the input to the converters independently so as to drawpower from each DC source at the MPP.

As can be appreciated, the benefit of the topology illustrated in FIGS.4A and 4B may be numerous. For example, the output characteristics ofthe serially connected DC sources, such as solar panels, need not match.Consequently, the serial string may utilize panels from differentmanufacturers or panels installed on different parts of the roofs (i.e.,at different spatial orientation). Moreover, if several strings areconnected in parallel, it may not be necessary that the strings match;rather each string may have different panels or different number ofpanels. This topology may also enhance reliability by alleviating thehot spot problem. As shown in FIG. 4B, the output of the shaded panel101/9 is 1.43 A, while the current at the output of the unshaded panelsis 6.25 A. This discrepancy in current when the components are seriesconnected may cause a large current being forced through the shadedpanel that may cause overheating and malfunction at this component.However, by the exemplary aspects of the topology shown, the inputvoltage may be set independently, and the power draw from each panel toits converter may be set independently according to the panel's MPP ateach point in time, the current at each panel may be independent on thecurrent draw from the serially connected converters.

It may be easily realized that since the power may be optimizedindependently for each panel, panels may be installed in differentfacets and directions in building-integrated photovoltaic (BIPV)installations. Thus, the problem of low power utilization in buildingintegrated installations may be solved, and more installations may nowbe profitable. The described system may also easily solve the problem ofenergy harvesting in low light conditions. Even small amounts of lightmay be enough to make the converters 305 operational, and they thenstart transferring power to the inverter. If small amounts of power areavailable, there may be a low current flow—but the voltage will be highenough for the inverter to function, and the power may indeed beharvested. According to embodiments, inverter 404 may include a controlloop 420 to maintain an optimal voltage at the input of inverter 404. Inthe example of FIG. 4B, the input voltage to inverter 404 may bemaintained at 400V by the control loop 420. The converters 305 may betransferring substantially all (e.g., >95%) of the available power fromthe solar panels to the input of the inverter 404. As a result, theinput current to the inverter 404 may be dependent only on the powerprovided by the solar panels and the regulated set, i.e., constant,voltage at the inverter input.

Conventional inverter 104, shown in FIG. 1 and FIG. 2 , may have a verywide input voltage to accommodate for changing conditions, for example achange in luminance, temperature and aging of the solar array. This maybe in contrast to inverter 404 that may be designed according toaspects. The inverter 404 does not utilize a wide input voltage and maybe therefore simpler to design and more reliable. This higherreliability may be achieved, among other factors, by the fact that theremay be no voltage spikes at the input to the inverter and thus thecomponents of the inverter experience lower electrical stress and maylast longer. When the inverter 404 may be a part of a circuit, the powerfrom the panels may be transferred to a load that may be connected tothe inverter. To enable the inverter 404 to work at its optimal inputvoltage, any excess power produced by the solar array, and not used bythe load, may be dissipated. Excess power may be handled by selling theexcess power to the utility company if such an option is available. Foroff-grid solar arrays, the excess power may be stored in batteries. Yetanother option may be to connect a number of adjacent houses together toform a micro-grid and to allow load-balancing of power between thehouses. If the excess power available from the solar array is not storedor sold, then another mechanism may be provided to dissipate excesspower. The features and benefits explained with respect to FIGS. 4A and4B stem, at least partially, from having inverter 404 control thevoltage provided at its input. Conversely, a design may be implemented,where inverter 404 controls the current at its input. Such anarrangement may be illustrated in FIG. 4C. FIG. 4C illustrates anembodiment where the inverter controls the input current. Power outputof each solar panel 101 may be maintained at the maximum power point forthe panel by a control loop within the corresponding power converter305. In the example shown in FIG. 4C, all of the panels may be exposedto full sun illumination and each solar panel 101 provides 200 W ofpower.

Consequently, the MPPT loop will draw current and voltage level thatwill transfer the entire 200 W from the panel to its associatedconverter. That is, the current and voltage controlled by the MPPT formthe input current Iin and input voltage Vin to the converter. The outputvoltage of the converter may be determined by the constant current setat the inverter 404, as will be explained below. The output voltage Voutwould then be the total power, i.e., 200 W, divided by the outputcurrent lout. As noted above, according to an embodiment, the inputcurrent to inverter 404 may be controlled by the inverter by way ofcontrol loop 420. For the purpose of this example, assume the inputcurrent is kept as 5 A. Since it is assumed that there may be tenserially connected power converters, each providing 200 W, the inputvoltage to the inverter 404 is 2000 W/5 A=400V. Thus, the currentflowing through each of the converters 101/I-101/10 may be 5 A. Thismeans that in this idealized example each of the converters provides anoutput voltage of 200 W/5

-   -   a. A=40V. Now, assume that the MPPT for each panel (assuming        perfect matching panels) controls the MPP voltage of the panel        to Vmpp=32V. This means that the input voltage to the inverter        would be 32V, and the input current would be 200 W/32V=6.25 A.

Consequently, similar advantages have been achieved by having inverter404 control the current, rather than the voltage. However, unlikeconventional art, changes in the output of the panels may not causechanges in the current flowing to the inverter, as that may be set bythe inverter itself. Therefore, inverter 404 may be designed to keep thecurrent or the voltage constant, then regardless of the operation of thepanels, the current or voltage to inverter 404 will remain constant.

FIG. 5 illustrates a distributed power harvesting system 50, accordingto other embodiments, using DC power sources. FIG. 5 illustratesmultiple strings 303 coupled together in parallel. Each of strings 303may be a series connection of multiple modules and each of the modulesincludes a DC power source 101 that may be coupled to a converter 305.The DC power source may be a solar panel. The output of the parallelconnection of the strings 303 may be connected, again in parallel, to ashunt regulator 506 and a load 504. The load 504 may be an inverter aswith the embodiments of FIGS. 4A and 4B. Shunt regulators automaticallymaintain a constant voltage across its terminals. The shunt regulator506 may be configured to dissipate excess power to maintain the inputvoltage at the input to the inverter 504 at a regulated level andprevent the inverter input voltage from increasing. The current whichflows through shunt regulator 506 complements the current drawn byinverter 504 in order to ensure that the input voltage of the invertermay be maintained at a constant level, for example at 400V.

By fixing the inverter input voltage, the inverter input current may bevaried according to the available power draw. This current may bedivided between the strings 303 of the series connected converters. Wheneach converter 305 includes a control loop 311 maintaining the converterinput voltage at the maximum power point of the associated DC powersource, the output power of converter 305 may be determined. Theconverter power and the converter output current together may determinethe converter output voltage. The converter output voltage may be usedby a power conversion circuit in the converter for stepping up orstepping down the converter input voltage to obtain the converter outputvoltage from the input voltage as determined by the MPPT.

FIG. 6 illustrates an illustrative example of DC-to-DC converter 305according to embodiments. DC-to-DC converters may be conventionally usedto either step down or step up a varied or constant DC voltage input toa higher or a lower constant voltage output, depending on therequirements of the circuit. However, in the embodiment of FIG. 6 theDC-DC converter may be used as a power converter, i.e., transferring theinput power to output power, the input voltage varying according to themaximum power point, while the output current being dictated by theconstant input voltage to inverter 304, 404, or 504. That is, the inputvoltage and current may vary at any time and the output voltage andcurrent may vary at any time, depending on the operating condition ofthe DC power sources. The converter 305 may be connected to acorresponding DC power source 101 (or 101) at input terminals 614 and616. The converted power of the DC power source 101 may be output to thecircuit through output terminals 610 and 612. Between the inputterminals 614 and 616 and the output terminals 610 and 612, theremainder of the converter circuit may be located that includes inputand output capacitors 620 and 640, back flow prevention diodes 622 and642 and a power conversion circuit including a controller 606 and aninductor 608.

The inputs 616 and 614 may be separated by a capacitor 620, which mayact as an open circuit to a DC voltage. The outputs 610 and 612 may bealso separated by a capacitor 640 that also acts as an open circuit toDC output voltage. These capacitors may be DC blocking or AC-couplingcapacitors that short circuit when faced with alternating current of afrequency, which may be selectable. Capacitor 640 coupled between theoutputs 610 and 612 may also operate as a part of the power conversioncircuit discussed below. Diode 642 may be coupled between the outputs610 and 612 with a polarity such that current may not backflow into theconverter 305 from the positive lead of the output 612. Diode 622 may becoupled between the positive output lead 612 through inductor 608, whichacts as a short for DC current and the negative input lead 614 with sucha polarity to prevent a current from the output 612 to backflow into thesolar panel 101.

The DC power source 101 may be a solar panel, solar cell, string orsolar panels or a string of solar cells. A voltage difference may existbetween the wires 614 and 616 due to the electron-hole pairs produced inthe solar cells of panel 101. Converter 305 may maintain maximum poweroutput by extracting current from the solar panel 101 at its peak powerpoint by continuously monitoring the current and voltage provided by thepanel and using a maximum power point tracking algorithm. Controller 606may include an MPPT circuit or algorithm for performing the peak powertracking. Peak power tracking and pulse width modulation, PWM, may beperformed together to achieve the desired input voltage and current. TheMPPT in the controller 606 may be any conventional MPPT, such as, e.g.,perturb and observe (P&O), incremental conductance, etc. However,notably, the MPPT may be performed on the panel directly, i.e., at theinput to the converter, rather than at the output of the converter. Thegenerated power may be then transferred to the output terminals 610 and612. The outputs of multiple converters 305 may be connected in series,such that the positive lead 612 of one converter 305 may be connected tothe negative lead 610 of the next converter 305 (e.g., as shown in FIG.4 a ).

In FIG. 6 , the converter 305 may be shown as a buck plus boostconverter. The term “buck plus boost” as used herein may be a buckconverter directly followed by a boost converter as shown in FIG. 6 ,which may also appear in the literature as “cascaded buck-boostconverter”. If the voltage is to be lowered, the boost portion may beshorted (e.g., FET switch 650 statically closed). If the voltage is tobe raised, the buck portion may be shorted (i.e., FET switch 630statically closed). The term “buck plus boost” differs from buck/boosttopology, which may be a classic topology that may be used when voltageis to be raised or lowered. The efficiency of “buck/boost” topology maybe inherently lower than a buck plus boost converter. Additionally, forgiven requirements, a buck/boost converter may need bigger passivecomponents than a buck plus boost converter in order to function.Therefore, the buck plus boost topology of FIG. 6 may have a higherefficiency than the buck/boost topology. However, the circuit of FIG. 6may have to continuously decide whether it may be bucking (operating thebuck portion) or boosting (operating the boost portion). In somesituations when the desired output voltage may be similar to the inputvoltage, then both the buck and boost portions may be operational.

The controller 606 may include a pulse width modulator, PWM, or adigital pulse width modulator, DPWM, to be used with the buck and boostconverter circuits. The controller 606 controls both the buck converterand the boost converter and determines whether a buck or a boostoperation is to be performed. In some circumstances both the buck andboost portions may operate together. That is, as explained with respectto the embodiments of FIGS. 4A and 4B, the input voltage and inputcurrent may be selected independently of the selection of output currentand output voltage. Moreover, the selection of either input or outputvalues may change at any given moment depending on the operation of theDC power sources. Therefore, in the embodiment of FIG. 6 the convertermay be constructed so that at any given time a selected value of inputvoltage and input current may be up converted or down converteddepending on the output requirement. In one implementation, anintegrated circuit (IC) 604 may be used that incorporates some of thefunctionality of converter 305. IC 604 may be a single ASIC able towithstand harsh temperature extremes present in outdoor solarinstallations. ASIC 604 may be designed for a high mean time betweenfailures (MTBF) of more than 25 years. However, a discrete solutionusing multiple integrated circuits may also be used in a similar manner.In the exemplary embodiment shown in FIG. 6 , the buck plus boostportion of the converter 305 may be implemented as the IC 604. Practicalconsiderations may lead to other segmentations of the system. Forexample, in one embodiment, the IC 604 may include two ICs, one analogIC, which handles the high currents and voltages in the system, and onesimple low-voltage digital IC, which includes the control logic. Theanalog IC may be implemented using power FETs that may alternatively beimplemented in discrete components, FET drivers, A/Ds, and the like. Thedigital IC may form the controller 606.

In the exemplary circuit shown, the buck converter includes the inputcapacitor 620, transistors 628 and 630, a diode 622 positioned inparallel to transistor 628, and an inductor 608. The transistors 628 and630 may each have a parasitic body diode 624 and 626, respectively. Inthe exemplary circuit shown, the boost converter includes the inductor608, which may be shared with the buck converter, transistors 648 and650, a diode 642 positioned in parallel to transistor 650, and theoutput capacitor 640. The transistors 648 and 650 may each have aparasitic body diode 644 and 646, respectively.

FIG. 7 illustrates another illustrative embodiment of a power converter305, according to embodiments. FIG. 7 highlights, among others, amonitoring and control functionality of a DC-to-DC converter 305,according to embodiments. A DC voltage source 101 is also shown in thefigure. DC-to-DC converter 305 is connected to DC voltage source 101through input 716, and connected to output capacitor 740 through output612. Portions of a simplified buck and boost converter circuit are shownfor converter 305. The portions shown include the switching transistors728, 730, 748 and 750 and the common inductor 708. Each of the switchingtransistors may be controlled by a power conversion controller 706.

The power conversion controller 706 includes the pulse-width modulation(PWM) circuit 733, and a digital control machine 743 including aprotection portion 737. The power conversion controller 706 may becoupled to microcontroller 790, which includes an MPPT algorithm 719,and may also include a communication module 709, a monitoring andlogging module 711, and a protection module 735.

A current sensor 703 may be coupled between the DC power source 101 andthe converter 305, and output of the current sensor 703 may be providedto the digital control machine 743 through an associated analog todigital converter 723. A voltage sensor 704 may be coupled between theDC power source 101 and the converter 305 and output of the voltagesensor 704 may be provided to the digital control machine 743 through anassociated analog to digital converter 724. The current sensor 703 andthe voltage sensor 704 may be used to monitor current and voltage outputfrom the DC power source, e.g., the solar panel 101. The measuredcurrent and voltage may be provided to the digital control machine 743and may be used to maintain the converter input power at the maximumpower point.

The PWM circuit 733 controls the switching transistors of the buck andboost portions of the converter circuit. The PWM circuit may be adigital pulse-width modulation (DPWM) circuit. Outputs of the converter305 taken at the inductor 708 and at the switching transistor 750 may beprovided to the digital control machine 743 through analog to digitalconverters 741, 742, so as to control the PWM circuit 733.

A random access memory (RAM) module 715 and a non-volatile random accessmemory (NVRAM) module 713 may be located outside the microcontroller 790but coupled to the microcontroller 790. A temperature sensor 779 and oneor more external sensor interfaces 707 may be coupled to themicrocontroller 790. The temperature sensor 779 may be used to measurethe temperature of the DC power source 101. A physical interface 717 maybe coupled to the microcontroller 790 and used to convert data from themicrocontroller into a standard communication protocol and physicallayer. An internal power supply unit 739 may be included in theconverter 305.

In various embodiments, the current sensor 703 may be implemented byvarious techniques used to measure current. In one embodiment, thecurrent measurement module 703 may be implemented using a very low valueresistor. The voltage across the resistor will be proportional to thecurrent flowing through the resistor. In another embodiment, the currentmeasurement module 703 may be implemented using current probes, whichuse the Hall Effect to measure the current through a conductor withoutadding a series resistor. After translating the current measurement to avoltage signal, the data may be passed through a low pass filter andthen digitized. The analog to digital converter associated with thecurrent sensor 703 may be shown as the A/D converter 723 in FIG. 7 .Aliasing effect in the resulting digital data may be avoided byselecting an appropriate resolution and sample rate for the analog todigital converter. If the current sensing technique does not utilize aseries connection, then the current sensor 703 may be connected to theDC power source 101 in parallel.

In one embodiment, the voltage sensor 704 uses simple parallel voltagemeasurement techniques in order to measure the voltage output of thesolar panel. The analog voltage may be passed through a low pass filterin order to minimize aliasing. The data may be then digitized using ananalog to digital converter. The analog to digital converter associatedwith the voltage sensor 704 may be shown as the A/D converter 724 inFIG. 7 . The A/D converter 724 has sufficient resolution to generate anadequately sampled digital signal from the analog voltage measured atthe DC power source 101 that may be a solar panel.

The current and voltage data collected for tracking the maximum powerpoint at the converter input may be used for monitoring purposes also.An analog to digital converter with sufficient resolution may correctlyevaluate the panel voltage and current. However, to evaluate the stateof the panel, even low sample rates may be sufficient. A low-pass filtermakes it possible for low sample rates to be sufficient for evaluatingthe state of the panel. The current and voltage data may be provided tothe monitoring and logging module 711 for analysis.

Temperature sensor 779 enables the system to use temperature data in theanalysis process. The temperature may be indicative of some types offailures and problems. Furthermore, in the case that the power sourcemay be a solar panel, the panel temperature may be a factor in poweroutput production.

The one or more optional external sensor interfaces 707 enableconnecting various external sensors to the converter 305. Externalsensors 707 may be used to enhance analysis of the state of the solarpanel 101, or a string or an array formed by connecting the solar panels101. Examples of external sensors 707 include ambient temperaturesensors, solar radiance sensors, and sensors from neighboring panels.External sensors may be integrated into the converter 305 instead ofbeing attached externally. In one embodiment, the information acquiredfrom the current and voltage sensors 703, 704 and the optionaltemperature and external sensors 707 may be transmitted to a centralanalysis station for monitoring, control, and analysis using thecommunications interface 709. The central analysis station is not shownin the figure.

The communication interface 709 connects a microcontroller 790 to acommunication bus. The communication bus can be implemented in severalways. In one embodiment, the communication bus may be implemented usingan off-the-shelf communication bus such as Ethernet or RS422. Othermethods such as wireless communications or power line communications,which could be implemented on the power line connecting the panels, mayalso be used. If bidirectional communication is used, the centralanalysis station may request the data collected by the microcontroller790. Alternatively or in addition, the information acquired from sensors703, 704, 707 may be logged locally using the monitoring and loggingmodule 711 in local memory such as the RAM 715 or the NVRAM 713.

Analysis of the information from sensors 703, 704, 707 enables detectionand location of many types of failures associated with power loss insolar arrays. Smart analysis can also be used to suggest correctivemeasures such as cleaning or replacing a specific portion of the solararray. Analysis of sensor information can also detect power lossescaused by environmental conditions or installation mistakes and preventcostly and difficult solar array testing.

Consequently, in one embodiment, the microcontroller 790 simultaneouslymaintains the maximum power point of input power to the converter 305from the attached DC power source or solar panel 101 based on the MPPTalgorithm in the MPPT module 719, and manages the process of gatheringthe information from sensors 703, 704, 707. The collected informationmay be stored in the local memory 713, 715 and transmitted to anexternal central analysis station. In one embodiment, themicrocontroller 790 may use previously defined parameters stored in theNVRAM 713 in order to operate converter 305. The information stored inthe NVRAM 713 may include information about the converter 305 such asserial number, the type of communication bus used, the status updaterate and the ID of the central analysis station. This information may beadded to the parameters collected by the sensors before transmission.

Converters 305 may be installed during the installation of the solararray or retrofitted to existing installations. In both cases,converters 305 may be connected to a panel junction connection box or tocables connecting the panels 101. Each converter 305 may be providedwith the connectors and cabling to enable easy installation andconnection to solar panels 101 and panel cables.

In one embodiment, physical interface 717 may be used to convert to astandard communication protocol and physical layer so that duringinstallation and maintenance, the converter 305 may be connected to oneof various data terminals, such as a computer or PDA. Analysis may thenbe implemented as software, which will be run on a standard computer, anembedded platform or a proprietary device.

The installation process of converters 305 may include connecting eachconverter 305 to a solar panel 101. One or more of sensors 703, 704, 707may be used to ensure that the solar panel 101 and the converter 305 maybe properly coupled together. During installation, parameters such asserial number, physical location and the array connection topology maybe stored in the NVRAM 713. These parameters may be used by analysissoftware to detect future problems in solar panels 101 and arrays.

When the DC power sources 101 are solar panels, one of the problemsfacing installers of photovoltaic solar panel arrays may be safety. Thesolar panels 101 may be connected in series during the day when theremay be sunlight. Therefore, at the final stages of installation, whenseveral solar panels 101 may be connected in series, the voltage acrossa string of panels may reach dangerous levels. Voltages as high as 600Vmay be common in domestic installations. Thus, the installer faces adanger of electrocution. The converters 305 that may be connected to thepanels 101 may use built-in functionality to prevent such a danger. Forexample, the converters 305 may include circuitry or hardware ofsoftware safety module that limits the output voltage to a safe leveluntil a predetermined minimum load may be detected. Only after detectingthis predetermined load does the microcontroller 790 ramps up the outputvoltage from the converter 305. Another method of providing a safetymechanism may be to use communications between the converters 305 andthe associated inverter for the string or array of panels. Thiscommunication, that may be for example a power line communication, mayprovide a handshake before any significant or potentially dangerouspower level may be made available. Thus, the converters 305 would waitfor an analog or digital release signal from the inverter in theassociated array before transferring power to inverter. The abovemethodology for monitoring, control and analysis of the DC power sources101 may be implemented on solar panels or on strings or arrays of solarpanels or for other power sources such as batteries and fuel cells.

Reference is now made to FIG. 8A, which illustrates graphically behaviorof power output in FIG. 2 from solar panels 101 (and which is input toinverter module 104) as a function of current in conventional system 10.Power increases approximately linearly until a current at which amaximum power point MPP may be found which may be some average over theMPP points of all connected solar panels 101. Conventional MPPT module107 locks (e.g., converges) on to the maximum power point.

Reference is now also made to FIG. 8B which illustrates graphicallypower input or power output versus output current from series/parallelconnected modules 302 or strings 303 (FIG. 3 ). It may be readily seenthat by virtue of control circuit 311 in modules 302, power as afunction of current output may be approximately constant. Similarly,power as a function of voltage output may be approximately constant. Itis desirable and it would be advantageous to have a system in whichmodules 302 and/or string 303 of FIG. 3 operate with the conventionalinverter 104 equipped with an MPPT module 107 of FIG. 2 . However, asshown in FIG. 8B, MPPT 107 does not have a maximum power peak (versuscurrent or voltage) on which to lock on to and MPPT circuit 107 maybecome unstable with varying or oscillating current/voltage at the inputof inverter module 104. In order to avoid this potential instability,according to a feature, a maximum power at an output voltage or currentat least for a time period may be output or presented to conventionalinverter module 104 equipped with MPPT module 107 according to variousaspects.

Reference is now made to FIG. 8C which illustrates in a simplified blockdiagram of a photovoltaic distributed power harvesting system 80including photovoltaic panels 101 a 101 d connected respectively topower converter circuits 305 a-305 d. Solar panel 101 together with itsassociated power converter circuit 305 forms photovoltaic module 302.Each converter 305 a-305 d adapts to the power characteristics of theconnected solar panel 101 a-101 d and transfers the power efficientlyfrom converter input to converter output. Each converter 305 a-305 dincludes control circuit 311 that receives a feedback signal from theinput from solar panel 101. Control circuit 311 may be a maximum powerpoint tracking (MPPT) control loop. The MPPT loop in converter 305 locksthe input voltage and current from each solar panel 10 la-101 d to itsoptimal power point (i.e., to converge on the maximum power point).

System 80 includes a series and/or parallel connection between outputsof strings 303 and the input of a conventional inverter 104 with anintegrated MPPT module 107. Inverter 104 with integrated MPPT module 107is designed to be connected directly to the outputs with series/parallelconnections of conventional solar panels 101 as in conventional system10 of FIG. 1 .

Referring back to FIG. 7 , MPPT algorithm 719 of microcontroller 790 inconverters 305 may, in various embodiments, provide a slight maximuminput power at a predetermined output voltage or current or conversionratio into MPPT 107. The input power into MPPT 107 may be maximized at apredetermined value of output voltage or current. In one embodiment, asshown in FIG. 8D), the maximum at the predetermined maximum power pointmay be very slight with a total variation of just a few percent toseveral percent over the entire input range of current or voltage ofinverter 104. In other embodiments, a circuit, 81 disposed betweenpanels 101 or strings 303 and inverter 104 may be used to present toMPPT module 107 with a maximum power point onto which to lock (e.g.,converge).

Reference is now made to FIG. 8E which illustrates an embodiment ofcircuit 81 for generating a maximum power point at the input of MPPTmodule 107 in configuration 80 (FIG. 8 ), according to an embodiment.Circuit 81 may be a power attenuator interposed betweenparallel-connected strings 303 and MPPT module 107. Circuit 81 mayinclude a non-linear current sink “f’ configured to draw a small amountof current at a particular voltage or voltage range from the DC powerline connecting strings 303 to MPPT module 107. The output of currentsink “f’ may be fed into the positive input of operational amplifier Al.The output of operational amplifier Al feeds the base of transistor T 1,the emitter of which may be connected and fed back to the negative inputof operational amplifier Al. The collector of transistor Tl connects tothe positive DC power line. The negative DC power line may be connectedto the emitter of transistor Tl through a shunt resistor Rs.

Reference is now made to FIG. 8F, which illustrates a simplified methodfor operating modules 302 and/or strings 303 with inverter 104 equippedwith an MPPT module 107. Reference is also made again to FIGS. 6 and 7 .The output voltage of power converter 305 is sensed (step 801) acrossoutput terminals 610 and 612. Control circuit 311 may be configured toset (step 803) the input power received at the input terminals 614/616to a maximum power for a predetermined output voltage point or voltagerange or at a predetermined output current point or current range. Thepredetermined values may be stored in memory 713 and/or 715 or may bereceived through communications interface 709. Away from thepredetermined output voltage or predetermined output current, thecontrol circuit may be configured to set (step 803) the input powerreceived at the input terminals to less than the maximum available power(i.e., decrease the input power in response to the difference betweenthe output current and the predetermined current increasing, andincrease the input power towards the maximum available power in responseto the difference between the output current and the predeterminedcurrent decreasing). In certain variations, the predetermined outputcurrent values may be selected such that the output power of module 302or string 303 is as shown in FIG. 8D. The predetermined output voltagevalues versus output power may be selected in a similar way. While FIG.8D illustrates one possible embodiment, other embodiments may presentMPPT module 107 with other output power versus current (or voltage)curves that have one or more local maximum to which the MPPT 107 cantrack and lock (e.g., converge). In this way, maximum power pointtracking circuit 107, if present, may stably track (step 805) thevoltage and/or current point or range. When a maximum is reached(decision block 807), MPPT tracking circuit 107 locks (step 809) ontothe power point (e.g., the “predetermined point” in FIG. 8D).

Reference is now made to FIG. 9 , which illustrates in a simplifiedblock diagram a photovoltaic distributed power harvesting system 90including photovoltaic panels 101 a 101 d connected respectively topower converter circuits 905 a-905 d. One solar panel 101 together withits associated connected power converter circuit 905 forms aphotovoltaic module 902. Each converter 905 a-905 d adapts to the powercharacteristics of the connected solar panel 905 a-905 d and transfersthe power efficiently from converter input to converter output. Eachconverter 905 a-905 d includes a control circuit 900 that receives afeedback signal from input sensor 904. Specifically, input currentsensors and/or voltage sensors 904 are used to provide the feedback tocontrol circuit 900. Control circuit 900 may also receive a signal fromoutput current and/or output voltage sensors 906.

Inverter 104 with integrated MPPT module 107 is designed to be connecteddirectly to the outputs with series/parallel connections of conventionalsolar panels 101 as in conventional system 10 of FIG. 1 .

Although photovoltaic modules 902 may be designed to be integrated withinverters 304 it may be advantageous that each panel module 902 may alsobe integrated with a respective conventional inverter (similar toinverter 104) between the converter 905 output and the seriallyconnected outputs of module 902 (not illustrated). System 90 includes aseries and/or parallel connection between outputs of strings 903 inputto a conventional inverter

a. 104 with an integrated MPPT module 107.

Reference is now made to FIG. 8G, which illustrates another method 821for operating modules 902, and/or strings 903 with inverter 104 equippedwith an MPPT module 107. In step 823, a scan is made by control circuit900 making a variation of the voltage conversion ratio between inputvoltage and output voltage (Vout) of a power converter circuit 905.During the variation, multiple measurements may be made (step 825) ofthe input and/or output power (e.g., by measuring input and outputcurrent and voltage) of converter 905 for different voltage conversionratios that are set by control circuit 900 during the variation. Thepower measurements made for each different voltage conversion ratio maythen be used to determine (step 827) the maximum power point of theconnected photovoltaic source. From the determination of the maximumpower point of the connected photovoltaic source, the voltage conversionratio for the maximum point may be used to set (step 829) the conversionratio for a continued operation of converter 905. The continuedoperation of converter 905 continues for a time period (step 831) beforeapplying another variation of the voltage conversion ratio in step 823.

Reference is now made to flow diagrams of FIGS. 9 a and 9 b , accordingto various aspects. Power converter 905 may control output voltage byvarying (step 811) the output voltage from power converter 905. Theinput voltage to power converter 905 may be maintained at the maximumpower point. The conversion ratio defined as the ratio of input voltageto output voltage may be varied or perturbed to slowly approach (step811) maximum power on the output terminals. The term “slowly” as usedherein is relative to the response time of MPPT circuit 107 associatedwith load 104. The conversion ratio or output voltage may be selected.

By adjusting the conversion ratio of the power converter, the efficiencyof the converter can be adjusted, thereby increasing or decreasing theoutput power for a received input power. Thus, in one example, while amaximum power point is maintained at the power converter input, theoutput can be adjusted to increase the output power to provide a maximumpower point for MPPT 107 (e.g., predetermined point in FIG. 8D)).

Since the output power from power converter 905 approaches slowlymaximum power, MPPT circuit 107 responds accordingly and locks onto theoutput voltage at maximum output power. Referring now to FIG. 9 b , inthe meantime MPPT circuit 107 associated with load 104 tracks the slowvariation of output power from photovoltaic modules 902. In FIG. 9 c , agraph is shown which indicates the slow variation of output power fromphotovoltaic modules 902, which varies typically over many seconds (DT).

According to various embodiments, the processes of 9 a and 9 b may beperformed in conjunction with other previously described embodiments tomove the maximum power point presented to the inputs of MPPT circuit107. For example, the maximum point illustrated in FIG. 8D) or (othermaximum point) may be shifted to a different current and/or voltage suchthat maximum power is maintained over changing power production andconversion conditions (e.g., light, temperature, faults, etc.) ofsystems 30/40/50/80/90. The rate of adapting the system (e.g., movingthe peak) is slower than the tracking rate of MPPT 107, such that theMPPT maintains lock (e.g., convergence) on the current/voltage/power atits input of inverter 104 within the power peak (e.g., the “maximumpoint” in FIG. 8D)).

Reference is now made to FIGS. 10A and 10B, which together illustrateanother process that allows systems 30/90 to be integrated with inverter104 equipped with MPPT circuit 107. In FIG. 10A, MPPT circuit 107perturbs (step 191) voltage or current across string 303. Controlcircuit 900 senses (step 195) the voltage or current perturbation ofMPPT circuit 107. Control circuit 900 via sensor 906 in step 197 slowlymaximizes output power at a particular voltage conversion ratio ofconverter 905. Input power from a photovoltaic panel 101 may bemaximized. In decision block 817, a maximum output power is beingreached and in step 193 MPPT 107 locks onto the maximum output power.

The articles “a”, “an”, as used hereinafter are intended to mean and beequivalent to “one or more” or “at least one”, For instance, “a directcurrent (DC) power source” means “one or more direct current (DC) powersources”.

Aspects of the disclosure have been described in terms of illustrativeembodiments thereof. While illustrative systems and methods as describedherein embodying various aspects of the present disclosure are shown, itwill be understood by those skilled in the art, that the disclosure isnot limited to these embodiments. Modifications may be made by thoseskilled in the art, particularly in light of the foregoing teachings.For example, each of the features of the aforementioned illustrativeexamples may be utilized alone or in combination or sub combination withelements of the other examples. For example, any of the above describedsystems and methods or parts thereof may be combined with the othermethods and systems or parts thereof described above. For example, oneof ordinary skill in the art will appreciate that the steps illustratedin the illustrative figures may be performed in other than the recitedorder, and that one or more steps illustrated may be optional inaccordance with aspects of the disclosure. It will also be appreciatedand understood that modifications may be made without departing from thetrue spirit and scope of the present disclosure. The description is thusto be regarded as illustrative instead of restrictive on the presentdisclosure.

1. An apparatus comprising: a power converter having input terminals andoutput terminals and being configured to convert input power receivedfrom a direct current (DC) power source at the input terminals to anoutput power at the output terminals; at least one input sensor coupledto the input terminals and configured to sense an input parameter whichincludes an input current or an input voltage; and a control circuitconfigured to maximize the input power to a maximum power point at theinput terminals based on the input parameter, wherein, for at least atime interval, the control circuit is configured to maintain the inputpower at the maximum power point and to set the output power of thepower converter to measurably less than the maximum power point, andafter the time interval, the control circuit is configured to maintainthe input power at the maximum power point and to set the output powerof the power converter to the maximum power point.
 2. The apparatus ofclaim 1, wherein the control circuit is configured to maintain the inputpower at the maximum power point with a first control loop operating ata first control frequency, and wherein the maximum power point ismeasurable by an external circuit with a second control loop operatingat a second control frequency that is greater than the first controlfrequency.
 3. The apparatus of claim 1, wherein the control circuit isconfigured to vary a conversion ratio between the input terminals andthe output terminals to maintain the input power at the maximum powerpoint and to set the output power to measurably less than the maximumpower point.
 4. The apparatus of claim 3, further comprising at leastone output sensor coupled to the output terminals, wherein the at leastone output sensor is configured to sense an output parameter includingan output current or an output voltage, and wherein, based on avariation of the input power or the output power, the control circuit isconfigured to vary the conversion ratio so that the input powerapproaches the maximum power point.
 5. The apparatus of claim 1, whereinthe control circuit is configured to track the output power.
 6. Theapparatus of claim 1, further comprising: a plurality of additionalpower converters having respective input terminals and respective outputterminals and being configured to convert input power received from arespective plurality of additional DC power sources, wherein the outputterminals of the power converter and the respective output terminals ofthe plurality of additional power converters are connected in series toform a serial string, and an external circuit, wherein the externalcircuit is operatively connected to the serial string, and wherein theexternal circuit is configured to track a variation of combined outputpower of the serial string.
 7. The apparatus of claim 6, furthercomprising a load including load input terminals and load outputterminals, wherein the load input terminals are configured to receivethe combined output power via the external circuit.
 8. The apparatus ofclaim 7, wherein the load includes: an inverter or a DC-to-DC powerconverter.
 9. The apparatus of claim 1, wherein the DC power sourceincludes at least one photovoltaic panel or at least one photovoltaiccell.
 10. The apparatus of claim 1, wherein the control circuit isconfigured to, for at least the time interval, maintain the input powerat the maximum power point and to set the output power of the powerconverter to measurably less than the maximum power point by adjustingan efficiency of conversion associated with the power converter.
 11. Theapparatus of claim 1, wherein the control circuit is configured to, forat least the time interval, maintain the input power at the maximumpower point and to set the output power of the power converter tomeasurably less than the maximum power point by changing a conversionratio of the power converter.
 12. The apparatus of claim 1, wherein thecontrol circuit is configured to set the input power to the maximumpower point for a predetermined output voltage value.
 13. The apparatusof claim 12, wherein the predetermined output voltage value comprises anoutput voltage point or an output voltage range.
 14. The apparatus ofclaim 12, wherein the predetermined output voltage value is received viaa communication interface.
 15. A method comprising: converting inputpower received from a direct current (DC) power source at an inputterminal to an output power at an output terminal; sensing an inputparameter including an input current or an input voltage; based on thesensed input parameter, maximizing the input power to a maximum powerpoint at the input terminal; setting, by a control circuit, the inputpower at the maximum power point and the output power to measurably lessthan the maximum power point for a time interval; and after the timeinterval, setting the output power equal to the maximum power point. 16.The method of claim 15, further comprising: varying a conversion ratiobetween the input terminal and the output terminal to maintain the inputpower at the maximum power point and to set the output power tomeasurably less than the maximum power point.
 17. The method of claim16, further comprising: sensing an output parameter including at leastone of an output current or an output voltage, wherein, based on avariation of the input power or the output power, varying the conversionratio so that the input power approaches the maximum power point.
 18. Amethod comprising: converting, by a power converter, input powerreceived from a direct current (DC) power source at an input terminal toan output power at an output terminal; sensing, at the input terminal,an input parameter including an input current or an input voltage; basedon the sensed input parameter, performing maximum power point trackingon the input terminal; and varying, by a control circuit and whileperforming maximum power point tracking on the input terminal, anefficiency of the power converter.
 19. The method of claim 18, furthercomprising: sensing, at the output terminal, an output parameterincluding an output current or an output voltage, wherein the varying ofthe efficiency of the power converter comprises at least one of:varying, based on a variation of the input power or the output power, aconversion ratio between the input terminal and the output terminal sothat the input power approaches a maximum power point; varying aconversion ratio between the input terminal and the output terminal tomaintain the input power at a maximum power point and to set the outputpower to measurably less than the maximum power point; or varying, basedon a comparison of the sensed output parameter and a predeterminedoutput value, a conversion ratio between the input terminal and theoutput terminal.
 20. The method of claim 19, further comprising:receiving the predetermined output value via a communication interface.